If you need a circular area you can do a periodical check (pleas don’t do it in tick) to check if the vector length between your player world location and the circle center (works even if it’s not 0,0,0) is larger than your desired radius.
If you need a square area you have to check each axys individually, for example:
100m x 100m square with center at 0,0,0
You can use this condition to kill the player:
If(Abs(PlayerWorldLocation.X) > 5000.0f || Abs(PlayerWorldLocation.Y) > 5000.0f)
(You can use ABS() to get the value without the sign, but you can do this only if the center is at 0, if you need a custom center you have to do four checks)